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<p>在类定义中，关键词 <a href="../keywords/static.html" title="cpp/keywords/static" class="mw-redirect"><tt>static</tt></a> 声明不绑定到类实例的成员。
</p><p>在类外定义中，它有不同含义：见<a href="storage_duration.html" title="cpp/language/storage duration">存储期</a>。
</p>
<h3><span class="mw-headline" id=".E8.AF.AD.E6.B3.95">语法</span></h3>
<table class="t-sdsc-begin">

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</td></tr>
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<td> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">static</span></span></span> <span class="t-spar">数据成员</span>
</td>
<td> (1)
</td>
<td class="t-sdsc-nopad">
</td></tr>



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<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">static</span></span></span> <span class="t-spar">成员函数</span>
</td>
<td> (2)
</td>
<td class="t-sdsc-nopad">
</td></tr>



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<div class="t-li1"><span class="t-li">1)</span> 声明静态数据成员。</div>
<div class="t-li1"><span class="t-li">2)</span> 声明静态成员函数。</div>
<h3><span class="mw-headline" id=".E8.A7.A3.E9.87.8A">解释</span></h3>
<p>类的静态成员不与类的对象关联：它们是具有<a href="storage_duration.html" title="cpp/language/storage duration">静态<span class="t-rev-inl t-since-cxx11"><span>或线程</span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span>存储期</a>的独立变量，或者常规函数。
</p><p><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">static</span></span></span> 关键词仅与静态成员在类定义中的声明一同使用，但不与该静态成员的定义一同使用：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">class</span> X <span class="br0">{</span> <span class="kw4">static</span> <span class="kw4">int</span> n<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// 声明（用 'static'）</span>
<span class="kw4">int</span> X<span class="sy4">::</span><span class="me2">n</span> <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span>              <span class="co1">// 定义（不用 'static'）</span></pre></div></div>
<p>类体内的声明不是定义，且可以将成员声明为具有（除 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span></span></span> 之外的）<a href="incomplete_type.html#.E4.B8.8D.E5.AE.8C.E6.95.B4.E7.B1.BB.E5.9E.8B" title="cpp/language/type">不完整类型</a>，包括该成员的声明所在的类型：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> Foo<span class="sy4">;</span>
<span class="kw1">struct</span> S
<span class="br0">{</span>
   <span class="kw4">static</span> <span class="kw4">int</span> a<span class="br0">[</span><span class="br0">]</span><span class="sy4">;</span> <span class="co1">// 声明，不完整类型</span>
   <span class="kw4">static</span> Foo x<span class="sy4">;</span>   <span class="co1">// 声明，不完整类型</span>
   <span class="kw4">static</span> S s<span class="sy4">;</span>     <span class="co1">// 声明，不完整类型（在其自身定义中）</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw4">int</span> S<span class="sy4">::</span><span class="me2">a</span><span class="br0">[</span><span class="nu0">10</span><span class="br0">]</span><span class="sy4">;</span> <span class="co1">// 定义，完整类型</span>
<span class="kw1">struct</span> Foo <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
Foo S<span class="sy4">::</span><span class="me2">x</span><span class="sy4">;</span>     <span class="co1">// 定义，完整类型</span>
S S<span class="sy4">::</span><span class="me2">s</span><span class="sy4">;</span>       <span class="co1">// 定义，完整类型</span></pre></div></div>
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<tr class="t-rev t-since-cxx11"><td>
<p>然而，若声明使用了 <a href="constexpr.html" title="cpp/language/constexpr"><tt>constexpr</tt></a><span class="t-rev-inl t-since-cxx17"><span> 或 <a href="inline.html" title="cpp/language/inline"><tt>inline</tt></a></span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span> 说明符，则必须声明该成员为具有完整类型。
</p>
</td>
<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td></tr>
</table>
<p>可用两种形式来指代类 <code>T</code> 的静态成员 <code>m</code>：限定名 <code>T::m</code>，或成员访问表达式 <code>E.m</code> 或 <code>E-&gt;m</code>，其中 <code>E</code> 分别是求值为 <code>T</code> 或 <code>T*</code> 的表达式。在同一类作用域中时，限定是不必要的：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> X
<span class="br0">{</span>
    <span class="kw4">static</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 声明</span>
    <span class="kw4">static</span> <span class="kw4">int</span> n<span class="sy4">;</span>    <span class="co1">// 声明</span>
<span class="br0">}</span><span class="sy4">;</span>
 
X g<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> X<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// 某个返回 X 的函数</span>
 
<span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    X<span class="sy4">::</span><span class="me2">f</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>  <span class="co1">// X::f 是静态成员函数的限定名</span>
    g<span class="br0">(</span><span class="br0">)</span>.<span class="me1">f</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// g().f 是指代静态成员函数的成员访问表达式</span>
<span class="br0">}</span>
 
<span class="kw4">int</span> X<span class="sy4">::</span><span class="me2">n</span> <span class="sy1">=</span> <span class="nu0">7</span><span class="sy4">;</span> <span class="co1">// 定义</span>
 
<span class="kw4">void</span> X<span class="sy4">::</span><span class="me2">f</span><span class="br0">(</span><span class="br0">)</span> <span class="co1">// 定义</span>
<span class="br0">{</span> 
    n <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// X::n 在此作用域可以仅以 n 进行访问</span>
<span class="br0">}</span></pre></div></div>
<p>静态成员遵循<a href="access.html" title="cpp/language/access">类成员访问规则（私有、受保护、公开）</a>。
</p>
<h4><span class="mw-headline" id=".E9.9D.99.E6.80.81.E6.88.90.E5.91.98.E5.87.BD.E6.95.B0">静态成员函数</span></h4>
<p>静态成员函数不关联到任何对象。调用时，它们无 <span class="t-c"><span class="mw-geshi cpp source-cpp">this</span></span> 指针。
</p><p>静态成员函数不能为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">virtual</span></span></span>、<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span></span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">volatile</span></span></span>。
</p><p>静态成员函数的地址可以存储在常规的<a href="pointer.html#.E5.87.BD.E6.95.B0.E6.8C.87.E9.92.88" title="cpp/language/pointer">函数指针</a>中，但不能存储于<a href="pointer.html#.E6.88.90.E5.91.98.E5.87.BD.E6.95.B0.E6.8C.87.E9.92.88" title="cpp/language/pointer">成员函数指针</a>中。
</p>
<h4><span class="mw-headline" id=".E9.9D.99.E6.80.81.E6.95.B0.E6.8D.AE.E6.88.90.E5.91.98">静态数据成员</span></h4>
<p>静态数据成员不关联到任何对象。即使不定义类的任何对象它们也存在。整个程序中只有一个拥有静态<a href="storage_duration.html" title="cpp/language/storage duration">存储期</a>的静态数据成员实例<span class="t-rev-inl t-since-cxx11"><span>，除非使用关键词 <a href="../keyword/thread_local.html" title="cpp/keyword/thread local"><tt>thread_local</tt></a>，该情况下每个线程都有一个具有线程存储期的该对象</span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span>。
</p><p>静态数据成员不能为 <span class="t-c"><span class="mw-geshi cpp source-cpp">mutable</span></span>。
</p><p>在命名空间作用域中，若类自身具有外部连接（即不是<a href="namespace.html#.E6.97.A0.E5.90.8D.E5.91.BD.E5.90.8D.E7.A9.BA.E9.97.B4" title="cpp/language/namespace">无名命名空间</a>的成员），则类的静态数据成员也具有<a href="storage_duration.html" title="cpp/language/storage duration">外部连接</a>。局部类（定义于函数内部的类）和无名类，包括无名类的成员类，不能拥有静态数据成员。
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx17"><td>
<p>静态数据成员可以声明为 <a href="inline.html" title="cpp/language/inline"><tt>inline</tt></a>。内联静态数据成员可以在类定义中定义，而且可以指定初始化器。它不需要类外定义：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> X
<span class="br0">{</span>
    <span class="kw1">inline</span> <span class="kw4">static</span> <span class="kw4">int</span> n <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
</td>
<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td></tr>
</table>
<h4><span class="mw-headline" id=".E5.B8.B8.E9.87.8F.E9.9D.99.E6.80.81.E6.88.90.E5.91.98">常量静态成员</span></h4>
<p>若整型或枚举类型的静态数据成员被声明为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span></span></span>（且非 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">volatile</span></span></span>），则它能以其中的每个表达式均为<a href="constexpr.html" title="cpp/language/constexpr">常量表达式</a>的<a href="initialization.html" title="cpp/language/initialization">初始化器</a>，直接在类定义内初始化：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> X
<span class="br0">{</span>
    <span class="kw4">const</span> <span class="kw4">static</span> <span class="kw4">int</span> n <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span>
    <span class="kw4">const</span> <span class="kw4">static</span> <span class="kw4">int</span> m<span class="br0">{</span><span class="nu0">2</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// C++11 起</span>
    <span class="kw4">const</span> <span class="kw4">static</span> <span class="kw4">int</span> k<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">const</span> <span class="kw4">int</span> X<span class="sy4">::</span><span class="me2">k</span> <span class="sy1">=</span> <span class="nu0">3</span><span class="sy4">;</span></pre></div></div>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<p>若<a href="../named_req/LiteralType.html" title="cpp/named req/LiteralType"><i>字面类型</i> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LiteralType)</span> </a>的静态数据成员被声明为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span></span></span>，则它必须以其中的每个表达式均为常量表达式的初始化器，直接在类定义内初始化：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> X <span class="br0">{</span>
    <span class="kw4">constexpr</span> <span class="kw4">static</span> <span class="kw4">int</span> arr<span class="br0">[</span><span class="br0">]</span> <span class="sy1">=</span> <span class="br0">{</span> <span class="nu0">1</span>, <span class="nu0">2</span>, <span class="nu0">3</span> <span class="br0">}</span><span class="sy4">;</span>        <span class="co1">// OK</span>
    <span class="kw4">constexpr</span> <span class="kw4">static</span> <a href="../numeric/complex.html"><span class="kw1472">std::<span class="me2">complex</span></span></a><span class="sy1">&lt;</span><span class="kw4">double</span><span class="sy1">&gt;</span> n <span class="sy1">=</span> <span class="br0">{</span><span class="nu0">1</span>,<span class="nu0">2</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// OK</span>
    <span class="kw4">constexpr</span> <span class="kw4">static</span> <span class="kw4">int</span> k<span class="sy4">;</span> <span class="co1">// 错误：constexpr static 要求初始化器</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
</td>
<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td></tr>
</table>
<p>若 const <span class="t-rev-inl t-since-cxx17"><span>非内联</span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span>静态数据成员<span class="t-rev-inl t-since-cxx11"><span>或 constexpr 静态数据成员</span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span>被 <a href="definition.html#ODR_.E5.BC.8F.E4.BD.BF.E7.94.A8" title="cpp/language/definition">ODR 式使用</a>，则仍需要命名空间作用域的定义，但它不能有初始化器。<span class="t-rev-inl t-since-cxx17"><span>此定义对于 <code>constexpr</code> 数据成员已弃用</span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span>。
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> X <span class="br0">{</span>
    <span class="kw4">static</span> <span class="kw4">const</span> <span class="kw4">int</span> n <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span>
    <span class="kw4">static</span> <span class="kw4">constexpr</span> <span class="kw4">int</span> m <span class="sy1">=</span> <span class="nu0">4</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">const</span> <span class="kw4">int</span> <span class="sy2">*</span>p <span class="sy1">=</span> <span class="sy3">&amp;</span>X<span class="sy4">::</span><span class="me2">n</span>, <span class="sy2">*</span>q <span class="sy1">=</span> <span class="sy3">&amp;</span>X<span class="sy4">::</span><span class="me2">m</span><span class="sy4">;</span> <span class="co1">// X::n 与 X::m 被 ODR 式使用</span>
<span class="kw4">const</span> <span class="kw4">int</span> X<span class="sy4">::</span><span class="me2">n</span>, X<span class="sy4">::</span><span class="me2">m</span><span class="sy4">;</span>       <span class="co1">// ……故需要定义</span>
<span class="kw4">constexpr</span> <span class="kw4">int</span> X<span class="sy4">::</span><span class="me2">m</span><span class="sy4">;</span>         <span class="co1">// （但 C++17 中的 X::m 不需要）</span></pre></div></div>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx17"><td>
<p>若静态数据成员被声明为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span></span></span>，则它隐含为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">inline</span></span></span> 且不必在命名空间作用域重声明。这种无初始化器的重声明（之前则为必要，如上所示）仍然得到容许，但已被弃用。
</p>
</td>
<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td></tr>
</table>
<h3><span class="mw-headline" id=".E5.BC.95.E7.94.A8">引用</span></h3>
<div class="t-ref-std-c++11">
<ul><li> C++11 standard (ISO/IEC 14882:2011): 
</li></ul>
<dl><dd><ul><li> 9.4 Static members [class.static]
</li></ul>
</dd></dl>
</div>
<div class="t-ref-std-c++98">
<ul><li> C++98 standard (ISO/IEC 14882:1998): 
</li></ul>
<dl><dd><ul><li> 9.4 Static members [class.static]
</li></ul>
</dd></dl>
</div>
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
<ul><li> <a href="storage_duration.html" title="cpp/language/storage duration"><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">static</span></span></span> 存储说明符</a>
</li></ul>

<!-- 
NewPP limit report
Preprocessor visited node count: 2377/1000000
Preprocessor generated node count: 5779/1000000
Post‐expand include size: 52709/2097152 bytes
Template argument size: 10888/2097152 bytes
Highest expansion depth: 18/40
Expensive parser function count: 0/100
-->

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